1. Field of the Invention
The present invention relates to a DC motor which is suitable for a vehicle starter.
2. Description of the Related Art
As a prior art, a starter is disclosed in JP-U-63-143040. The motor of this starter has a face-contact commutator. The face-contact commutator has a surface perpendicular to an armature shaft to be in contact with a pair of brushes. There is a difference between this face-contact commutator and a common cylindrical commutator in that: it is necessary for the cylindrical commutator to have an axial marginal length for receiving the brushes, while it is not necessary for the face-contact commutator to have the same. In other words, the face-contact commutator is effective to shorten the axial length of the motor.
The motor disclosed in the publication has compression coil springs for biasing the brushes against a commutator. Compression coil springs are disposed at the rear end of brushes opposite to the commutator. If the spring constant of the coil spring is lowered and the initial spring is increased to provide a sufficient spring force for a long time until the brushes are worn away, the total length of springs is increased. That is, although the face-contact commutator shortens the length of the armature, the total length of the motor can not be reduced.
If the spring force is increased to shorten its length, the springs may be fatigued shortly due to heat of the motor.
As disclosed in JP-A-7-194066, a resinous holder holding a brush holder thereon is fastened to a bracket by a bolt, and the bracket together with a torque sensor unit disposed thereon is also fastened to a flange of motor casing by bolts. Through holes are formed in the bracket and the flange and female screw holes are formed in the torque sensor unit. Because the inside diameter of the through holes is larger than the outside diameter of the bolts, clearances are formed between the bolts and the bracket's through holes. As a result, the bracket may move in the clearances, and this makes the circumferential positioning difficult.
Even after the motor casing and the brush holder have been assembled, the bracket may move within the clearance due to vibration. This changes circumferential position of the brush holder relative to the yoke.
In addition, the torque sensor and the motor casing with the bracket interposed therebetween necessitate to align three through holes thereof with each other.